In 1974, Hawking predicted that a black hole can emit radiation with a temperature characteristic of its gravitational field strength at the event horizon. This process can be viewed as spontaneous vacuum pair production with the subsequent splitting of this pair at the event horizon by the gravitational field leaving the other to escape. The idea that primordial black holes could exhibit radiation with a characteristic temperature led to Unruh's subsequent prediction that an accelerated observer would experience a similar radiation with a temperature proportional to the proper acceleration. Unruh's result is both fundamental and intriguing as it opened up the possibilities of observing such vacuum related phenomena with accelerations generated in the laboratory. Subsequent work by Fulling and Davies showed that a single accelerated mirror rather than an observer can result in an altering of the energy density of vacuum leading to the emission of photons.
A related problem of transformations of vacuum fields was undertaken prior to both Hawking's black hole prediction and the work of Unruh and Davies by Moore who considered the problem of a quantized electromagnetic field in a dynamic cavity. Moore showed that photons can be created by the effect of moving cavity mirrors on the zero point field fluctuations. Following Moore's seminal work, a number of calculations related to the effects of rapidly changing boundary conditions on a massless field obeying a covariant wave equation followed. These efforts considered the cases of harmonically driven mirrors and also predicted photon creation from vacuum with various squeezing effects.
The importance of Unruh's work, coupled to the various treatments of electromagnetic vacuum with dynamic boundary conditions, has led to ideas on how to observe the general effect of photon creation from the vacuum state in the laboratory. In general, these proposals all focused on creating photons from highly accelerated particles or through non-adiabatic changes in boundary conditions that could lead to measurable signals. Unruh's seminal paper relates the observer temperature to the proper acceleration by:
                    T        =                  ah          ck                                    (        1        )            where h is Plank's constant, c is the speed of light in vacuum, k is Boltzmann's constant and a is the proper acceleration. Based on equation (1), accelerations as large as 1023 m/s2 only lead to temperatures comparable to those associated with the cosmic background radiation.
The first idea for observing the Unruh effect was due to Unruh himself who suggested using a hydrodynamic analogue of the Schwarzchild metric with a quantized phonon field 12. This system had theoretical value beyond the suggested experimental measurements in that it provided a physical system to study the insensitivity of black hole evaporation on the exact form of the dispersion relations at high frequencies. However, the calculations showed that effective phonon temperatures of 1K required velocity gradients of 100 m/s per Angstrom, far beyond the capabilities of normal fluids.
Following Unruh's work on fluids, Bell and Leinaas suggested using the equilibration of electron spin polarization as a measure of the local proper frame temperature. In the case of linear accelerations, the time scale was glacial while for orbiting electrons the effect is highly complicated by orbital magnetic fields and the Sokolov-Ternov effect. Another effect relying on stored electrons was due to Rogers who suggested the use of a Penning trap coupled to a microwave cavity where photons emitted by the Unruh effect would lead to nonzero photon occupation numbers in certain cavity modes. This ingenious idea unfortunately predicts temperatures of 2-3K which are difficult to discern from stray mode coupling effects. Darbinyan et. al. proposed to use channeling phenomena in crystals to observe Unruh signals emanating from the scattering of vacuum by the transversely accelerated electrons. This proposal has the distinction of having the largest achievable accelerations with values as large as 1031 m/s2 for the ultra-relativistic particles. Unfortunately, the process behind the emission, Compton scattering of vacuum photons, is swamped by the enormous Brehmsstrahlung radiation predicted by the Bethe-Heitler formula.
In 1989, Yablonovich proposed combining the accelerated mirror effect of Fulling and Davies with ultra-fast material response in semiconductors. Using a laser produced plasma front created though either real or virtual photoconductivity in semiconductors results in an effective acceleration of 1021 m/s2 and an Unruh temperatures of ˜4K. Calculations based on dynamic changes in dispersion relations estimated this type of dynamic Casimir effect version of the Unruh process could result in infrared emission powers of 10−9 W. These powers take place over timescales of less than a 10−12 s and therefore lead to emitted energies of a fraction of one infrared photon per experimental event. Again, the expected signals from the experimental scenario fall far from the requirements for measurable signals, proving the existence of the Unruh effect and connecting laboratory physics to processes occurring in black holes smaller than the size of an atom.
The most recent proposal for measuring the Unruh effect in the laboratory is due to Chen and Tajima who suggest taking advantage of ongoing developments in petawatt lasers to create violent electron accelerations as large as 1026 m/s2. These types of accelerations are two orders of magnitude larger than those in plasma wakes and occur at the driving field frequencies as opposed to the slower natural modes of the plasma. This proposal predicts that petawatt lasers could result in Unruh energies which are four orders of magnitude weaker than the expected strong Larmor emission but with temporal and spatial signatures that are significantly different, allowing for detection. In addition, the authors suggest the possible use of coherent X-rays generated by free electron lasers to boost the signal to a value comparable to the Larmor energy. This approach has the highest acceleration values of any of the effects except for the channeling experiments of Darbinyan et. al. and does not suffer from Brehmsstrahlung radiation due to the interaction of charged particles with dense matter. Based on the combination of signal separation as well as overall background signal strengths, the proposal by Chen and Tajima remains the most promising approach to measuring the direct effect of acceleration of single electrons on the vacuum modes.
A plasmon is a density wave of charge carriers which form at the interface of a conductor and a dielectric. Plasmons determine, to a degree, the optical properties of conductors, such as metals. Plasmons at a surface can interact strongly with the photons of light, forming a polariton. Plasmon excitations at interfaces with dimensions comparable to or significantly smaller than the wavelength of excitation do not propagate and are localized. In ionic materials, phonons can produce a negative dielectric response and result in phonon-polaritons. Small scale dimensions lead to localized plasmon-polariton and phonon polaritons.
Localized surface plasmons have been observed since the time of the Romans, who used gold and silver nanoparticles to create colored glass objects such as the Lycurgus Cup (4th Century A.D.). A gold sol in the British museum, created by Michael Faraday in 1857, is still exhibiting its red color due to the plasmon resonance at ˜530 nm. In more recent times, localized plasmons have been observed on rough surfaces and in engineered nanostructures and have led to the observation and exploitation of Surface Enhanced Raman Scattering (SERS) and new tunable plasmon structures with potential applications in biology and medicine.
Despite the large number of suggested schemes for observing the Unruh effect and the related non-adiabatic transformations of quantized electromagnetic vacuum modes, the process of popping a measurable number of photons out of the vacuum remains elusive to this day. This paper describes a novel electrodynamic system using localized surface plasmons embedded in amplifying media, which can result in subpicosecond bursts of photons being out of the vacuum with pulse energies two or more orders of magnitude above the background spontaneous emission in the same time interval.